diff --git a/Include/internal/pycore_ceval.h b/Include/internal/pycore_ceval.h
index 64fb4034669..a357bfa3a26 100644
--- a/Include/internal/pycore_ceval.h
+++ b/Include/internal/pycore_ceval.h
@@ -124,7 +124,7 @@ _PyEval_Vector(PyThreadState *tstate,
             PyObject *kwnames);
 
 extern int _PyEval_ThreadsInitialized(void);
-extern PyStatus _PyEval_InitGIL(PyThreadState *tstate, int own_gil);
+extern void _PyEval_InitGIL(PyThreadState *tstate, int own_gil);
 extern void _PyEval_FiniGIL(PyInterpreterState *interp);
 
 extern void _PyEval_AcquireLock(PyThreadState *tstate);
diff --git a/Include/internal/pycore_pylifecycle.h b/Include/internal/pycore_pylifecycle.h
index daf7cb77dcc..c6750986857 100644
--- a/Include/internal/pycore_pylifecycle.h
+++ b/Include/internal/pycore_pylifecycle.h
@@ -63,7 +63,7 @@ extern void _PyArg_Fini(void);
 extern void _Py_FinalizeAllocatedBlocks(_PyRuntimeState *);
 
 extern PyStatus _PyGILState_Init(PyInterpreterState *interp);
-extern PyStatus _PyGILState_SetTstate(PyThreadState *tstate);
+extern void _PyGILState_SetTstate(PyThreadState *tstate);
 extern void _PyGILState_Fini(PyInterpreterState *interp);
 
 extern void _PyGC_DumpShutdownStats(PyInterpreterState *interp);
diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 4bb54c93b0a..4e9e13457a9 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -547,7 +547,9 @@ interp_create(PyObject *self, PyObject *args, PyObject *kwds)
         return NULL;
     }
 
+    PyThreadState_Swap(tstate);
     PyThreadState_Clear(tstate);
+    PyThreadState_Swap(save_tstate);
     PyThreadState_Delete(tstate);
 
     _PyInterpreterState_RequireIDRef(interp, 1);
diff --git a/Python/ceval_gil.c b/Python/ceval_gil.c
index 7581daa55b5..d70abbc2760 100644
--- a/Python/ceval_gil.c
+++ b/Python/ceval_gil.c
@@ -447,7 +447,7 @@ init_own_gil(PyInterpreterState *interp, struct _gil_runtime_state *gil)
     interp->ceval.own_gil = 1;
 }
 
-PyStatus
+void
 _PyEval_InitGIL(PyThreadState *tstate, int own_gil)
 {
     assert(tstate->interp->ceval.gil == NULL);
@@ -466,8 +466,6 @@ _PyEval_InitGIL(PyThreadState *tstate, int own_gil)
 
     // Lock the GIL and mark the current thread as attached.
     _PyThreadState_Attach(tstate);
-
-    return _PyStatus_OK();
 }
 
 void
diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index b5c7dc5da59..0ec29846b08 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -576,44 +576,33 @@ init_interp_settings(PyInterpreterState *interp,
         interp->feature_flags |= Py_RTFLAGS_MULTI_INTERP_EXTENSIONS;
     }
 
-    /* We check "gil" in init_interp_create_gil(). */
+    switch (config->gil) {
+    case PyInterpreterConfig_DEFAULT_GIL: break;
+    case PyInterpreterConfig_SHARED_GIL: break;
+    case PyInterpreterConfig_OWN_GIL: break;
+    default:
+        return _PyStatus_ERR("invalid interpreter config 'gil' value");
+    }
 
     return _PyStatus_OK();
 }
 
 
-static PyStatus
+static void
 init_interp_create_gil(PyThreadState *tstate, int gil)
 {
-    PyStatus status;
-
     /* finalize_interp_delete() comment explains why _PyEval_FiniGIL() is
        only called here. */
     // XXX This is broken with a per-interpreter GIL.
     _PyEval_FiniGIL(tstate->interp);
 
     /* Auto-thread-state API */
-    status = _PyGILState_SetTstate(tstate);
-    if (_PyStatus_EXCEPTION(status)) {
-        return status;
-    }
+    _PyGILState_SetTstate(tstate);
 
-    int own_gil;
-    switch (gil) {
-    case PyInterpreterConfig_DEFAULT_GIL: own_gil = 0; break;
-    case PyInterpreterConfig_SHARED_GIL: own_gil = 0; break;
-    case PyInterpreterConfig_OWN_GIL: own_gil = 1; break;
-    default:
-        return _PyStatus_ERR("invalid interpreter config 'gil' value");
-    }
+    int own_gil = (gil == PyInterpreterConfig_OWN_GIL);
 
     /* Create the GIL and take it */
-    status = _PyEval_InitGIL(tstate, own_gil);
-    if (_PyStatus_EXCEPTION(status)) {
-        return status;
-    }
-
-    return _PyStatus_OK();
+    _PyEval_InitGIL(tstate, own_gil);
 }
 
 
@@ -657,10 +646,7 @@ pycore_create_interpreter(_PyRuntimeState *runtime,
     }
     _PyThreadState_Bind(tstate);
 
-    status = init_interp_create_gil(tstate, config.gil);
-    if (_PyStatus_EXCEPTION(status)) {
-        return status;
-    }
+    init_interp_create_gil(tstate, config.gil);
 
     *tstate_p = tstate;
     return _PyStatus_OK();
@@ -2099,28 +2085,21 @@ new_interpreter(PyThreadState **tstate_p, const PyInterpreterConfig *config)
         return _PyStatus_OK();
     }
 
-    PyThreadState *tstate = _PyThreadState_New(interp,
-                                               _PyThreadState_WHENCE_INTERP);
-    if (tstate == NULL) {
-        PyInterpreterState_Delete(interp);
-        *tstate_p = NULL;
-        return _PyStatus_OK();
-    }
-    _PyThreadState_Bind(tstate);
-
+    // XXX Might new_interpreter() have been called without the GIL held?
     PyThreadState *save_tstate = _PyThreadState_GET();
-    int has_gil = 0;
+    PyThreadState *tstate = NULL;
 
     /* From this point until the init_interp_create_gil() call,
        we must not do anything that requires that the GIL be held
        (or otherwise exist).  That applies whether or not the new
        interpreter has its own GIL (e.g. the main interpreter). */
+    if (save_tstate != NULL) {
+        _PyThreadState_Detach(save_tstate);
+    }
 
     /* Copy the current interpreter config into the new interpreter */
     const PyConfig *src_config;
     if (save_tstate != NULL) {
-        // XXX Might new_interpreter() have been called without the GIL held?
-        _PyThreadState_Detach(save_tstate);
         src_config = _PyInterpreterState_GetConfig(save_tstate->interp);
     }
     else
@@ -2142,11 +2121,14 @@ new_interpreter(PyThreadState **tstate_p, const PyInterpreterConfig *config)
         goto error;
     }
 
-    status = init_interp_create_gil(tstate, config->gil);
-    if (_PyStatus_EXCEPTION(status)) {
+    tstate = _PyThreadState_New(interp, _PyThreadState_WHENCE_INTERP);
+    if (tstate == NULL) {
+        status = _PyStatus_NO_MEMORY();
         goto error;
     }
-    has_gil = 1;
+
+    _PyThreadState_Bind(tstate);
+    init_interp_create_gil(tstate, config->gil);
 
     /* No objects have been created yet. */
 
@@ -2165,16 +2147,14 @@ new_interpreter(PyThreadState **tstate_p, const PyInterpreterConfig *config)
 
 error:
     *tstate_p = NULL;
-
-    /* Oops, it didn't work.  Undo it all. */
-    if (has_gil) {
+    if (tstate != NULL) {
+        PyThreadState_Clear(tstate);
         _PyThreadState_Detach(tstate);
+        PyThreadState_Delete(tstate);
     }
     if (save_tstate != NULL) {
         _PyThreadState_Attach(save_tstate);
     }
-    PyThreadState_Clear(tstate);
-    PyThreadState_Delete(tstate);
     PyInterpreterState_Delete(interp);
 
     return status;
diff --git a/Python/pystate.c b/Python/pystate.c
index f0c5259967d..e18eb0186d0 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -1454,6 +1454,7 @@ void
 PyThreadState_Clear(PyThreadState *tstate)
 {
     assert(tstate->_status.initialized && !tstate->_status.cleared);
+    assert(current_fast_get(&_PyRuntime)->interp == tstate->interp);
     // XXX assert(!tstate->_status.bound || tstate->_status.unbound);
     tstate->_status.finalizing = 1;  // just in case
 
@@ -2150,7 +2151,7 @@ _PyGILState_Fini(PyInterpreterState *interp)
 
 
 // XXX Drop this.
-PyStatus
+void
 _PyGILState_SetTstate(PyThreadState *tstate)
 {
     /* must init with valid states */
@@ -2160,7 +2161,7 @@ _PyGILState_SetTstate(PyThreadState *tstate)
     if (!_Py_IsMainInterpreter(tstate->interp)) {
         /* Currently, PyGILState is shared by all interpreters. The main
          * interpreter is responsible to initialize it. */
-        return _PyStatus_OK();
+        return;
     }
 
 #ifndef NDEBUG
@@ -2170,8 +2171,6 @@ _PyGILState_SetTstate(PyThreadState *tstate)
     assert(gilstate_tss_get(runtime) == tstate);
     assert(tstate->gilstate_counter == 1);
 #endif
-
-    return _PyStatus_OK();
 }
 
 PyInterpreterState *